CA1054704A - Drop-out compensator - Google Patents

Abstract

APPLICATION FOR PATENT
OF
WAYNE RAY DAKIN
FOR
DROP-OUT COMPENSATOR

ABSTRACT OF THE DISCLOSURE
In the art of recording video signals, apparatus for drop-out compensation by detecting "missing" positive or negative going half cycles of recorded carrier signals includes logic for generating a "new" half cycle of appropriate polarity to replace individual missing half cycles which is inserted into the original recorded signal train. If a plurality of consecutive half cycles are missing, memory circuits supply signal information that approximates the information contained in the missing half cycles.

Description

1 ¦ BACKGROUND OF THE INVENTION

2 Field of the Invention

3 This invention relates to the recorded video art and,

4 more specifically, to a system for detecting and compensating for the loss of signals in the video recovered from the 6 recording medium. The absence of either positive or negative 7 going half cycles from the carrier signal is detected. Loss 8 of individual half cycles result in the generation of a new half cycle of the appropriate polarity and its insertion in the carrier. If several consecutive half cycles are lost, 11 informational content of the signal is retained and is applied 12 to the system until the carrier is restored.

14 Description of the Prior Art In the prior art, the recording and playback of the 16 high-frequency, FM encoded signals which transmit video informa-lq tion and which may be of the order of 7.0 MHz, required the 18 creation of special magnetic recording techniques and special 19 ¦ problems developed. Different proprietary systems have been evolved, such as increasing the linear speed of the tape 21 relative to a stationary head or using a plurality of rotating 22 heads that rotate at relatively high speed at substantially 23 right angles to a tape moving linearly. These systems have been 24 used successfully to record and play back black and white as 25 well as color television transmissions. However, whenever less 26 than 100 percent of the original signal is recorded or reproduced 27 for transmission to the display, the quality of the picture 28 suffers. This is especially so if color television is 29 involved.
The eye is extremely sensitive to drastic changes in the 31 visual impression made to the eye no matter how fleeting. For 32 example, a missing half cycle in an FM recorded carrier would . ' ' ~$
. -2-1 ¦ have the effect of drastically reducing the frequency of 2 ¦ the modulated signal at that instant. In many video reeording 3 ¦ systems, a decrease in the frequency represents a brlghtening 4 ¦ of the picture. A missing half cycle has the effect of

5 ¦ drastically lowerin~ the fre~uency and is responded to by the

6 I television displa~ system as an immediate and sudden, dis-

7 eontinuous lightening of the picture, sometimes ealled a "blooming" effect, w~Lich produces an intense white spot in what 9 eould be an otherwise "dark" region of a picture.
10 ¦ Sueh an instantaneous change in the brightness of a 11 ¦ portion ~f the television display system is very disconcerting 12 ¦ to the viewer and may be likened to static or ~pops" and 13 ¦ "elieks" emanating from a high fidelity sound recording. The 14 ¦ white spot on the television screen is caused by the intensity 15 ¦ amplifier being overdriven into a saturation range. The 16 persistence of the screen maintains the spot for a relatively lq long time and the effect is eompounded by the persistance of the ~8 image in the eye, disproportionately exaggerating the 19 ef~ect.
If several eonsec~tive half eyeles of earrier are lost, 21 either due to imperfections in the recording medium or from 22 malfunctions in the recording or playback system, portions of 23 the video picture may be lost and "streaks n will appear in the 24 display. This, too, is disconeerting to the viewer and represents a problem that must be solved to provide a commercially 26 acceptable video reeording and reproducing system suitable for 27 the consumer. `

~ 1054704 1 I Summary of the Invention I
2 ¦ According to the invention, the recorded carrier signal i 5 ¦ continuall~ examined by delaying the carrier signal by 4 ¦ approximately 1/2 cycle at the center fre~uency and continuously 5 ¦ comparing the delayed signal against the undelayed signal. Both 6 ¦ signals are fed through a logic network which detects the "absence"
7 in the undelayed signal of either a positive or negative going

8 ¦ half cycle. The logic network inserts a half cycle of proper

9 ~ polarity to replace any "missing" single half cycles in the

10 ¦ carrier.

11 ¦ Replacing single half c~cles generally is sufficient '

12 I to prevent the over driving of the intensity amplifier and

13 ¦ thereby minimizes the formation of discrete white spots on the

14 screen. ~owever, if more than one consecutive half cycle of the

15 ¦ 9ame polarity is missing, either a more complex detector

16 employing a greater delay would be needed or additional circuits lq must be provided.
1 Accordingly, with the present detector, if 1 the system detects the absence of more than one half cycle, a 2 memory circuit, which is coupled to and tracks the FM dis-2 criminator output, is signalled to retain the "last" value of 2 the information signal. This last value is then supplied to the 2 display system until the information flow is restored, as 2 determined by the missing cycle detector.
2 Retaining the latest value known to be "good" until 2 the signal is restored assumes that information changes more or 27 less continuously and that there is no catastrophic loss of 28 signal and only brief interruptions which are limited perhaps to 29 within a single line of a single field of a single frame.
Retaining the same information value in these cases would have 31 a negligible effect on the viewer.

~ _4_ Thus, the invention is used in combinatlon with means for generating a frequency modulated signal train representative of desired information, and relates to apparatus for detecting and replacing occasional missing half cycles of the train comprising: detecting means responsive to the absence of a half cycle in the signal train; pulse generating means responsive to the detecting means for generating a substitute half cycle pulse corresponding to an isolated non-consecutive missing half cycle in the signal train; and combining means coupled to the pulse generating means and the means for generating a signal train for combining the substitute half cycle pulse with the signal train in the proper timed relation-ship to produce a continuous sequential output signal train in which substitute half cycle pulses maintain the frequency of the output signal train within predetermined limits.
In another aspect the invention relates to the combination of first means for generating an original signal train having a frequency modulation representative of desired information from an applied carrier signal; means for delaying the signal train substantially n/2 cycles, where "n" is an o-dd integer; logic means coupled to the first means and the delaying means for logically combining the delayed signal train and its complement with the original signal train and its complement to provide a first output corresponding to positive half cycles and a second output corresponding to negative half cycles; a first one-shot circuit responsive to the first output of the logic means for generating half cycle pulses; a second one-shot circuit responsive to the second output of the logic means for generating half cycle pulses; and means for logically combining the outputs of the first and second one-shot circuits with the original signal train and its complement mb ~ ~ - 4a -for generating a continuous sequential signal train at a frequency within a predetermined range of frequencies.

mb/l~ - 4b -1 I BRIEF DESCRIPTIO~ OF T~IE DRAWINGS
2 ¦ ~he novel features which are believed to be 3 , characteristic of the invention, ~oth as to organization and 4 method of operation, together with further objects and advantages thereof, will be better understood from the following 6 description considered in connection with the accompanying q drawings in which several preferred embodiments of the invention 8 are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a 11 definition of the limits of the invention.
12 FIG. 1 is a block diagram of the conventional prior art 13 system;
14 FIG. 2 is a block diagram of the over-all system 15 incorporating the present invention;
16 FIG. 3 is a logical diagram illustrating how a "missing"

17 ne~ative or positive going half cycle is detected and a ~newn

18 half cycle of the proper polarity is inserted;

19 ` FIG. 4 is a timing diagram illustrating the ~aveforms

20 at the various pointsjof the circuit described in connection

21 with FIG. 3; and - -2 FIG. 5 is a logical diagram of a memory network to 23 retain information.
2~
DESCRIPTION OF THE PREFERRED EMBODIMENTS
26 The recording of video signals either graphically, 27 magnetically, or on ~hemoplastic deformable materials is 28 usually accomplished under controlled environments. For 29 example, the velocity of the medium with respect to the head is 30 maintained at some predetermined magnitude in order to maintain 31 frequency and phase relationships between the signals recorded.
32 Similarly, on playback, the relative veloclty of tne 1 medium and the playback transduc~r must be carefully controlled 2 so that the frequency and timing informa~ion which has been 3 recorded can be recovered without.substantial de~radation.
4 Unfortunately, neither the medium nor the video playback sxstem is of the same caliber as the ~ideo transcr~bin~ equipment, or 6 the original video input signal,and hence, there is a specific 7 need for insuring and protecting against any loss of the 8 carrier signal.
Referring now to FIG 1, there is shown a prior art video playback system 10, which contains the necessary mechanical 11 and electronic equipment for detacting the recorded -rideo signal.
12 Connected to the output of the video playback system 10 is a 13 video amplifier 11 which amplifies the recorded video signal and 14 feeds a hard limiter 12. The hard limiter 12 essentially performs the functions of a zero-crossing detector. The 1 substantially square wave output of the hard limiter 12 will 17 have a repetition frequency dependent upon the informational 18 content of the video signal being recorded, generally varying 19 between 5.6 MHz and 8.4 MHz. The output of the hard limiter 12 20 is fed to a FM discriminator which extracts the video information 21 from the carrier. The ~ideo information signal is applied to a

22 video display system 14 which is the utilization device observed

23 by the viewer.

24 The composite video signal, by definition, is adapted to

25 produce a lighter scene as the carrier frequency decreases. It

26 can be appreciated, therefore, by reviewing the waveforms

27 associated with the output of the hard limiter 12 that, should

28 a half cycle of the carrier be missing for any reason, the

29 instantaneous value of the frequency will decrease substantially

30 since the frequency of the output signal is the reciprocal

31 function of the period.

, ' . '' ~ ` ~054704 1 The absence of either a positive cr a negative going 2 half cycle will have the effect of substantially increasing the 3 period and reducing the apparent or instantaneous frequency.
4 A brlght, white spot or blooming of the picture will result in the 6 video display system 14.
6 The missing half cycle could be the result of defects .7 in the recording medium, defective video playback system 10, 8 defective video amplifier 11, or, for any other reason, a missed zero crossing detector output from the hard limiter 12. The objectionable blooming effect as displayed by the video display 11 system 14 will be the same. l li Referring now to FIG. 2, there is shown a block diagram ~3 which ideally illustrates the principle of the present invention 14 and which is arranged to aetect the absence of a poSitlVe or 15 negative going half cycle. A "new" half cycle of the proper .
16 polarity is generated and is then ninserted" into the carrier 17 signal feeding the FM discriminator 13. The video playback 18 system 10 feeding the video amplifier 11 and the hard limiter 12 19 are similar ln function to that described in connection with 20 FIG. 1.
21 However, the output of the hard limiter 12.is fed through 22 an appropriate delay (not shown) to a summing network 15 and to 23 a half cycle detector 16. The half cycle detector 16 detects 24 the absence of either a negative or a positive going half cycle from the hard limiter l2, and in response thereto, drives a half 26 cycle generator 17, which generates a half cycle of the proper 27 polarity. The half cycle is then fed into the summing network 28 15 in the proper timed relationship with the signal from the 29 hard limiter 12. The summing network 15 combines the outputs from the hard limiter 12 with the outputs from the half cycle 31 generator 17 and feeds the FM discriminator 13.

32 The.emb~diment of ~IG. 2 presupposes only occasional 1 I losses of single half cycles during an extended playback 2 i transmission. An alternative approach to the problem could .3 I utilize a memory circuit in place of the half cycle generator 4 17 ~Ihich would continuously monitor the signal content of the transmission and would be operable in re.sponse to signals 6 from the half cycle detector 16 signifying a missing half 7 cycle to continue to provide to the display system the last 8 "good" value of the information until an infor~ation flow . is restored, which would also be signalled by the half cycle detector 15.
11 Referring now to FIG. 3, there is shown a logic circuit 12 which is a preferred mechanization of the block diagram of 13 FIG. 2, and which will detect and replace occasional missing 14 half cycles. The operation of the logic diagram of FIG. 3 will best be understood by referring also to the squared 16 waveforms illustrated in FIG. 4, which are correlated to 17 selected output points of the logic diagram, and will more 18 fullyillustrate how a "missing" positive or negative going 14 half cycle is detected and how a new half cycle of the proper sense is generated and inserted into the carrier signal train 21 which feeds the FM discriminator 13, as illustrated in FIG. 2.
22 In FIG. 3, there is shown the hard limiter 12 which is 23 connected to gate 20 which inverts the output from the hard 24 limiter 12. It is assumed that the input signal to the gate 20 at different times will lack a positive going half cycle and 26 a negative going half cycle and that the output of gate 20 is 27 represented by the squared off waveform A in FIG. 4.
28 A review of waveform A shows a series of half cycles fro~
29 tl through-tl3. At the interval t5-t6 a positive half cycle ~30 is assumed to be "missing" followed by "normal" half cycles 31 ~ from t7 through tg~ The positive going half cycle at tg should 32 I be followed by a negative going half cycle at tlo, which, in this . , . I

~ -8-example, is "missing". The negative going half cycle at tl2 is "normal". By way of review therefore, waveform A has a missing positive half cycle at the intervals t5-t6 and a missing negative half cycle at the intervals tlo-tll.
The output of inverting gate 20 is directed to the input of a delay line 22, a gate 24 and a gate 26, which each receive the signal A as one of their inputs. Since the carrier requency involved can range between 5.6 MHz to 8.4 MHz, in this embodiment the delay to be caused by the delay line 22 is selected to be 1/2 cycle at the center frequency of 7.0 MHz, and is therefore approximately 1/2 cycle over the frequency band of interest.
It will be understood that the choice of 1/2 cycle where the delay 22 i8 intended to detect and signal missing half cycles with a minimum of delay. Obviously, the delay could be selected to equal any odd number of half cycles and would function to detect the loss of an occasional half cycle.
The output of the delay line 22 is illustrated as Curve B in FIG. 4 which is a signal train that is delayed a 1/2 cycle from that of curve A. The ou'put of the delay line 22, Curve B, is fed to a gate 28 that inverts the input signal to generate Curve B to the input of gate 24.
"NAND" ~ates have been employed in a mechanization of the preferred embodiment. The output signal of gate 24 AB illustrates a positive going transition of a signal at times t2, t4, t8 and tlo. To retain logical consistency throughout, and to retain the conventlon that a positive ' signal represents a "1" or true signal while a negative signal represents a "O", or false signal, the negative going signal output of one shot 30 will be identified as Ql-Gate 26, which simply inverted the waveform A, gener-ates waveform A, which is applied to the inputs of gates 32, 34, mb/lc~ g 1 ~ and 36. The output of the delay line 22 is also applied to the 2 input of gate 32. Gate 32, therefore, receives B and A to . . -- -- .
3 produce an output curve of BA. BA is fed as the input signal to 4 Q2 monostable multivibrator or one-shot 38 which is adapted to generate a positive going half cycle whenever the input signal 6 is positive going. Retaining the conventions heretofore set 7 out, the signal output of the one shot 38 is identified as Q2.
Whenever the input signal of BA is going positive, as at tl, 9 t3, t5, tg and tl3, one shot 38 generates a positive signal, such as at times t3, tS~ tg and tl3.
11 The positive half cycles generated by the one shot 38, 12 as shown in curve Q2 are fed as a second input of gate 36. .
13 The-output of gate 36 is A Q2- which in this example, is a 14 single, negative going pulse between times t5 and t6.
.15 The output of the one shot 30, Ql is applied to one .
16 input of gate 40. Curve A, is fed to the other input gate 40. .
17 The output of gate 40, A ~ and represents, as shown in FIG. 4, 18 a half cycle to replace the "missing" negative half cycle between 19 times tlO ànd tll.
A Ql and A Q2 are both fed to the inputs of gate 42, 21 which generates the reconstituted composite curve illustrated 22 in FIG. 4 as output 42, which is then fed to the FM discr~minator 23 13. Because of the extensive use of logical circuits, it will 24 be appreciated that the actual mechanization.of the invention 26 need not follow the diagram of FIG. 2, but rather any logically 26 e~uivalent circuit can generate the composite signal in the 27 correcting circuit of FIG. 3.
28 By way of review, it can be appreciated therefore, that 29 the inccming carrier signal is continuously being compared to a delayed version of itself, to determine the absence of 31 either a positive or a negative going half cycle in the incoming 32 signal train. In the event there are no signal discontinuities then the output of gate 40 will be equivalent to the signal A, which merely represents a phase reversal of A as shown in FIG. 4. The output of the final NAND gate 42 will then be equivalent to A.
It can be appreciated therefore, that the logical circuit illustrated in FIG. 3, causes the Q2 one shot 38 to generate a positive going half cycle in the event of a "missing" positive half cycle and the Ql one shot 30 to generate a signal which ultimately provides a negative going half cycle in the event of a "missing" negative half cycle in the original signal train.
The "missing" half cycles are inserted when the comparison of A with B indicates that such insertion is necessary. Since the Ql one shot 30 and the Q2 one shot 38 are generating signal trains which generally correspond to the original signal, it can be appreciated that a substan-tially fast acting monostable multivibrator is preferred.
Obviously the output generated by either of the two single shot multivibrators 30 and 38 will, in all probability neither be precisely identical in width to the original missing half cycles, nor will they contain or convey the same frequency modulation. However, no great signal discontinuities will be caused by the occasional synthesis of a "missing" half cycle.
Other circuits which are logically equivalent to the embodiment of the FIG. 3 will be able to substantially reduce the whitening or blooming effect of missing half cycles by preventing the frequency from being effectively halved. Other mechanizations according to the teaching of FIG. 2 will also reduce the deleterious effects of "missing"
half cycles and persons skilled in the art will be able to devise yet other, alternative embodiments without departing from the teachings of the invention.

mb/~

Thus far, there has been shown apparatus for countering ,.

mb/'.~ lla -. .- , 1 the catastrophic effect of a missing pulse in the carrier signal 2 on the display. The illustrated correcting circuits of the 3 pxesent invention "restore" missin~ carrier half cycles, but 4 add no "information" to replace the information that is lost.
~ Further, the mechanîzation of FIG. 3 is unable to cope 6 with the loss of signal for more than a half cycle.
7 ~n analysis of FIG. 4 would indicate that the loss of 8 two consecutive positive or negative going half cycles would not 9 result in the replacement o~ both of them but rather only the first. Therefore, in the absence of a mechanization of FIG. 2 11 that could recognize the loss of any number of half cycles, it is 12 desirable to have additional circuitry that can be called upon 13 to prevent discontinuity in the display, should a plurality of 14 half cycles be lost.
Turning next to FIG. 5, there is shown in block form, 16 an arrangement which can help to prevent the effects of such 17 discontinuities in the flow of information. The correcting ~rl8 circuits of FIG. 3~here designated generally by the reference 19 numeral 50, continuously supply an output signal from gate 42 20 which is applied through a delay 51, to an FM discriminator 21 circuit 52, which is substantially identical to the FM
22 discriminator 13 of FIG. 2. The FM discriminator 52 extracts the 23 information that has been modulated upon the carrier and this 24 information output signal is applied both to an output amplifier 54 and to a Sample and Hold network 5-6.
26 The Sample~and Hold network 56 also receives signal 2~ inputs from the correcting circuits 50, including si~nals 28 representing A, A, Ql,iQ2 and Q2 since it can be detêrmined when 29 half cycles are missing from'the states of-these signals. The Sample and Hold network 56 also receives, on a continuous basis, 31 the information output signal of the FM discriminator 52.
32 Whenever the correcting circuits 50 are called upon~to . 1054704 1 I more than one missing half cycle the most recent value of the 2 information output signal from the FM discriminator 52 is retained 3 in the Sample and Hold circuit 56. The Sample and Hold output 4 is then applied to the amplifier 54 in the succeeding time ~ intervals.
6 ~hen the carrier signal is restored, the correcting 7 circuits 50 again transmit the complete carrier. The FM
discriminator 52 information output signal is then credible and 9 "updates" the sampling cîrcuits of the Sample and Hold network 56 which then merely "track" the output signal. Since the 11 amplifier 54 receives both the output of the FM discriminator 52 12 and the Sample and Hold network 54, generally the inputs will be 13 identical. Whene~er there is a ~loss" of FM information,the 14 Sample and Hold network 54 will continue to apply an information signal which will be an adequate approximation for a portion 16 of a line of the picture.
17 Clearly, a multiplicity of circuits are known and 18 available to perform the Sample and Hold function under the control 19 of the correcting circuits 50. It is sufficient that the circumstances of their use be specified as hereinabove.
21 It is also within the scope of the present invention to 22 merely detect the absence of a half cycle and, rather than 23 generating a replacement half cycle, merely utilize a Sample 24 and Hold network to avoid inf~nmation loss.
Thus, there has been shown apparatus for eliminating 26 the objectionable white spots or "blooms" on the playback of 27 a recorded video signal. Occasional "missing" half cycles 28 are detected and half cycles of proper polarity are gen`erated 29 and inserted into the transmission. This prevents wide discontinuous excursions of the information signal fxom the 31 FM discriminator which would occur if a half cycle of the 32 carrier signal were lost.

l , If more than one consecutive half cycle were to be 2 lost, sample and hold circuits, which conti~uously monitor and 3 track the discrLminator output, apply to the display device, 4 the value of the last information s;gnal prior to the detection 5 of the missing pulses. Th stored value is then retained and 6 applied to the display system until the flow of information 7 from the discriminator is restored.
8 Accordingly, the scope of the invention should be 9 limited only by the claims appended hereto.

,51 ~0 . ' 22 . .

228 ''` ' ' ''' "'

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination with means for generating a frequency modulated signal train representative of desired information, apparatus for detecting and replacing occasional missing half cycles of the train comprising:
(a) detecting means responsive to the absence of a half cycle in the signal train;
(b) pulse generating means responsive to said detecting means for generating a substitute half cycle pulse corresponding to an isolated non-consecutive missing half cycle in the signal train; and (c) combining means coupled to said pulse generating means and the means for generating a signal train for combining said substitute half cycle pulse with the signal train in the proper timed relationship to produce a continuous sequential output signal train in which substitute half cycle pulses maintain the frequency of said output signal train within predetermined limits.

2. A combination according to claim 1 wherein said detecting means include means for delaying the signal train for substantially n/2 cycles, where "n" is an odd integer to produce a delayed signal train and means for comparing said delayed signal train with the undelayed signal train to determine that a half cycle is missing from the signal train.

3. The combination of claim 1 wherein said pulse generating means include first means responsive to a missing positive going half cycle in the signal train for generating a substitute half cycle pulse corresponding thereto; and second means responsive to a missing negative going half cycle in the signal train for generating a substitute half cycle corresponding thereto and wherein said combining means combine said substitute half cycle pulses with a signal train derived from the signal train in the proper timed relationship, thereby producing a continuous sequential signal train in which substitute half cycle pulses maintain the frequency of the signal train within predetermined limits.

4. A combination according to claim 3 wherein said first means include a first one-shot circuit for generating half cycle pulses, and said second means include a second one-shot circuit for generating half cycle pulses.

5. In a video display system, the method of compensating for occasional interruption of the video carrier signal otherwise resulting in a white spot comprising the steps of:
(1) splitting the carrier signal into at least two signal trains;
(2) delaying one of said signal trains relative to the other by at least ? cycle at the center frequency;
(3) comparing the delayed and undelayed signal trains to determined the absence of a transition between predetermined positive and negative signal limits, cor-responding to a missing non-consecutive positive or negative half cycle;
(4) generating substitute half cycle pulses of appropriate polarity and duration to correspond to missing half cycles; and (5) combining said substitute half cycle pulses with said signal train at the appropriate time to produce a composite signal train having substitute half cycles of proper polarity and duration in place of missing non-consecutive half cycles.

6. Apparatus for compensating for carrier signal dropouts in playing back a recorded video trans-mission, comprising in combination:
a. input means for receiving a raw playback signal;
b. dropout detecting means coupled to said input means including delaying-comparing means for comparing a first signal corresponding to the raw play-back signal with a second signal corresponding to the raw playback signal delayed by a predetermined amount, to determined the absence of isolated, non-consecutive half cycles in the raw signal; and c. generation and insertion means coupled to said dropout detecting means for creating a half cycle signal of appropriate polarity and for combining said created half cycle signal with said first signal in replacement of a detected absent half cycle signal, whereby said combination provides a derived processed playback carrier signal free from signal dropouts.

7. In combination, first means for generating a signal train having a frequency modulation representative of desired information;
second means for detecting isolated missing positive going half cycles in said signal train;
third means responsive to said second means for generating a substitute half cycle pulse, corresponding to each missing positive going half cycle;
fourth means for detecting isolated missing negative going half cycles in said signal train;

fifth means responsive to said fourth means for generating a substitute half cycle pulse corresponding to each missing negative going half cycle; and sixth means coupled to said first, third and fifth means, producing a continuous, sequential signal train at a frequency within a predetermined range of frequencies from the signal train and said substitute half cycle pulses.

8. In combination:
first means for generating an original signal train having a frequency modulation representative of desired information from an applied carrier signal;
means for delaying said signal train substantially n/2 cycles, where "n" is an odd integer;
logic means coupled to said first means and said delaying means for logically combining said delayed signal train and its complement with said original signal train and its complement to provide a first output cor-responding to positive half cycles and a second output corresponding to negative half cycles;
a first one-shot circuit responsive to said first output of said logic means for generating half cycle pulses;
a second one-shot circuit responsive to said second output of said logic means for generating half cycle pulses; and means for logically combining the outputs of said first and second one-shot circuits with the original signal train and its complement for generating a continuous sequential signal train at a frequency within a predetermined range of frequencies.

9. A system for preventing the blooming of a video display resulting from the absence of isolated positive or negative half cycles in a carrier signal, the combination comprising:
first means for generating a frequency modulated signal train from an applied carrier signal;
detecting means coupled to said first means for responding to the absence of occasional isolated half cycles in the carrier signal;
second means coupled to said detecting means and responsive to isolated, non-consecutive missing positive going half cycles in said signal train for generating a half cycle pulse, corresponding thereto;
and responsive to isolated non-consecutive missing negative going half cycles in said signal train for generating a half cycle pulse, corresponding thereto; and third means for combining the pulse output of said second means in the proper timed relationship with the first means output for producing a continuous sequential signal train at a frequency within a predetermined carrier frequency range.